It's just working it's way down the fashion ladder that's how these things go.
I for one love my flannel and try to mix it in my wardrobe as much as possible.
Is flannel plaid? Or just any button up long sleeve shirt?
It's a thicker, material version of a plaid shirt. The best way to describe it is just thicker/"fuzzuer" although I still don't think that's the right word.